1. Field of Invention
This invention relate to directional couplers in general and more particularly to directional couplers that have a small overall size.
2. Description of Related Art
Directional couplers are used in a variety of applications in the RF and microwave frequency range. FIG. 1 shows a schematic diagram of a prior art directional coupler 20 including a pair of coupled circuit lines 21 and 22 located between ground planes 23 and 24. This configuration is referred to as stripline. The circuit lines 23 and 24 are buried within a dielectric material, which commonly is a printed circuit board. The ground planes are one factor that determines the impedance and coupling of the circuit lines. The directional coupler 20 has four ports, an input port 25, an output port 26, a forward coupled port 27 and a reverse coupled port 28. An input signal or power applied to the input port 25 will go mainly to the output port 26. A portion of the input signal will be electromagnetically coupled to circuit line 22 and appear mostly at forward coupled port 27. A very small portion of the signal will go to the coupled reverse port 28.
The electrical signal coupled to the forward and reverse ports depends upon the coupled circuit line characteristic impedance and the coupling between the lines. Directivity is a measure of the directional coupler differentiation between ports.
Printed circuit boards have a dielectric material constant around 2.5. The low dielectric constant causes the overall size of the device to be large when designed for a given circuit line impedance.
A current unmet need exists for a directional coupler that is smaller with good electrical performance and that is low in cost to manufacture.